1. Field of the Invention
The present invention relates to a method for forming a buffer layer to grow gallium nitride single crystals, and more particularly to a method for forming a buffer layer to grow gallium nitride single crystals, comprising forming a buffer layer as a nanoporous vertical layer on the interface between a sapphire substrate and gallium nitride, to reduce dislocation defect density.
2. Description of the Related Art
The rapid development of information and communication technologies has brought about rapid development of communication technologies to transfer ultrahigh-speed and mass-storage signals. In accordance with increasing demand for personal cellular phones, satellite communications, military radars and communication repeaters of wireless-communication techniques, the demand for high-speed and high-power electric devices requiring micrometer and millimeter wave bandwidth of ultrahigh-speed information and communication systems gradually increases.
In particular, Group III-V nitride-based compound semiconductors composed of GaN, GaInN mixed crystals, AlGaInN mixed crystals, AlGaN mixed crystals, etc., are direct-transition semiconductor materials that have superior physical properties such as large energy gap, high thermal chemical stability and high electric saturation speed, and are widely utilized in a variety of applications including short/long wavelength ranges of photodiodes as well as high-frequency and high-power electric devices.
Gallium nitride (GaN)-based single crystal semiconductors are composed of single crystalline gallium nitride (GaN) films formed on a hetero-substrate by vapor-phase growth methods such as organic metal chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE). Of these, HVPE is suited to growing thick films with a size of several to several hundred nanometers (μm). In particular, the HVPE method enables several millimeters of bulk growth, depending on substrate conditions.
Meanwhile, a sapphire (Al2O3) substrate which has a hexagonal system, e.g., gallium nitride, is inexpensive and stable at high temperatures, and is generally used as the hetero-substrate, on which gallium nitride single crystals are grown. Sapphire is useful for growing gallium nitride thin films. However, sapphire is unsuitable for use in growing gallium nitride thick films in that the large difference (about 16%) in lattice constant between sapphire and gallium nitride, and the large difference (about 36%) in thermal expansion coefficient therebetween causes generation of inner strain in the nitride layer and dislocation of the single crystal hetero-substrate.
Such a dislocation phenomenon progresses in a crystal growth direction and threading dislocation is thus propagated onto the growth surface, thus causing crystallinity of nitride semiconductor substrates. As a result, electrical properties are deteriorated and the substrate is warped due to the generated inner strain. In particular, when crystals are grown on the hetero-substrate, the presence of the inner strain causes the substrate to warp in the process of cooling. The warp is still present on the free standing plate, in which the hetero-substrate is removed. Accordingly, when a layer-structure for devices is grown by polishing the warped substrate, uniform composition distribution cannot be obtained and lithographic processes cannot be thus evenly performed. For this reason, growth of layer-structures for devices disadvantageously causes a considerably low process yield. In an attempt to address the dislocation defect between sapphire/gallium nitride single crystal laminates, technologies such as epitaxial lateral overgrowth (ELOG), pendeo-epitaxy, similar thereto and void-assisted separation (VAS) to vertically grow gallium nitride were developed and have been employed. Such a technology is a method wherein a GaN thin film is primarily grown on a sapphire substrate, is then patterned using an inorganic material or a mask material such as SiO2, Sn or W, and is subjected to exposure and developing processes, and GaN is secondarily grown thereon.
However, the use of these methods does not enable the dislocation defect density between the sapphire substrate and the gallium nitride layer to be decreased to 5×105-6/cm2 or less. In addition, these methods involve use of artificial masks, inducing stress into the growth film, thus allowing the film to be warped and cracked, when crystals are grown to a thickness of 100 μm or higher, and finally causing the film to be broken. For this reason, high-quality gallium nitride single crystal substrates having a thickness of 350 μm or higher cannot be secured.
Accordingly, there is a demand in the art for methods for growing high-quality gallium nitride single crystal substrates that reduce tensile stress generated by the difference in thermal expansion coefficient between gallium nitride and the sapphire substrate, grow a gallium nitride layer to a thickness of 1 micrometer (μm) to several millimeters (mm) suitable for commercialization without causing cracks, and reduce the lattice constant difference to improve crystallinity.